Descartes. The Cogito. Cogito.
But there is a deceiver of supreme power and cunning who is deliberately and constantly deceiving me; and let him deceive me as much as he can, he will never bring it about that I am nothing so long as I think that I am something. So after considering everything very thoroughly, I must finally conclude that this proposition, I am, I exist, is necessary true whenever it is put forward by me or conceived in my mind [AT VII, p. 25].
I noticed that while I was trying thus to think everything false, it was necessary that I, who was thinking this, was something. And observing this truth ‘I am thinking, therefore I exist’ was so firm and sure that all the most extravagant suppositions of the sceptics were incapable of shaking it, I decided that I could accept it without scruple as the first principle of the philosophy I was seeking [AT VI 32].